When designing a spectacle lens including a region having progressively changing refractive power such as a progressive region, a meridian (principal vision line) where movement of an eye of a spectacle wearer from distance vision to near vision is assumed is defined on the lens and then an area of clear vision is set based on the defined meridian. Note that the area of clear vision is a region with less warping and blurs of images where astigmatism of transmission performance is suppressed to a certain value or less.
When designing a spectacle lens of such a type, it is practically impossible to secure a wide range of the area of clear vision over an entire area from a distance portion to a near portion in a uniform manner. For example, in a progressive region between the distance portion and near portion, a width of the area of clear vision (width in the horizontal direction) is narrowed as compared to that in the distance portion or near portion as a side effect of progressive variation in refractive power.
A general distance-near vision lens having progressive refractive power places more priority on distance vision and thus a distance portion is designed to be large, thereby resulting in a short progressive region. In this manner, the distance-near vision lens having progressive refractive power has a short progressive region length (length along a meridian within the progressive region). Therefore, refractive power is designed to change with a steep gradient from the distance portion to near portion.
Generally, the steeper the refractive power change is in the progressive region, the narrower a width of the area of clear vision is in the progressive region. Therefore, the distance-near vision lens having progressive refractive power has a narrow width of the area of clear vision in the progressive region. For example, when a spectacle wearer of the distance-near vision lens having progressive refractive power moves the eyes at a relatively fast speed by nodding or shaking the head up and down or to left and right, the spectacle wearer may feel strong uncomfortablesness after perceiving warping of images due to aberration in the progressive region as shaking (especially a side part in the progressive region). Therefore, the distance-near vision lens having progressive refractive power is not suitable for a spectacle wearer who uses the progressive region frequently.
Meanwhile, an intermediate-near vision lens with more focus on intermediate vision is known (e.g. JP 2008-65358 A). In the intermediate-near vision lens, as compared to the distance-near vision lens having progressive refractive power, a progressive region is designed to be longer and to extend in art upper side of the lens, thus allowing for refractive power in the progressive region to change gradually. This allows for suppressing aberration in a side part of the progressive region while simultaneously ensuring a wide width of the area of clear vision in the progressive region. This reduces warping and blurs of images in the progressive region and thus achieves superior performance for indoor use where intermediate vision and near vision are mainly used.